9 notebooks cost $8.01. Which equation would help determine the cost of 15 notebooks?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. If we let $x$ represent the cost of 15 notebooks, we have the following proportion: $\dfrac{x}{15}$ We have to pay $8.01 for 9 notebooks, and that can be written as a proportion: $\dfrac{\$8.01}{9}$ Since the price per notebook stays the same, these two proportions are equivalent. $\dfrac{x}{15} = \dfrac{\$8.01}{9}$